The present invention relates to circuits which receive electromagnetic radiation, and particularly to integrated circuits which perform RF receiver functions with extremely low net power consumption.
There are many uses for low-powered RF receivers. A particular area of need is for short-range data links. Due to the legal constraints of spectrum allocation, many such data links must use extremely small RF signal levels. Moreover, in many cases the transceivers used for such communication must operate with minimal power drain, including minimal standby power. It is very difficult to satisfy the objectives of low power consumption while also providing adequate sensitivity and noise rejection.
Many of the innovative teachings set forth herein will be described in the context of a system for short-range wireless data communication between a base station and a portable low-power module. Such systems can be extremely useful in many contexts, such as control of personnel access to secure facilities, livestock management and other forms of automated manufacturing, medical monitoring of inpatients, theft control, and others described below.
However, such a system is subject to many constraints. If the portability of the portable stations is to be maximized, the battery weight must be small. This means that the power consumption of the portable module--in active or in standby mode--must be exceedingly low. Moreover, many possible applications are highly cost-sensitive.
In many such applications, the size and weight of the portable module is extremely sensitive. A module which is merely transportable will not suffice. For example, pagers and portable radios have often had weights of 10 ounces or more, and volumes of 10 cubic inches or more. If modules of this size were used (for example) for patient identification in a hospitals, the patients would unload such cumbersome objects as quickly as possible, by any means possible. Similarly, in many applications such large modules could not be used for inventory control, since there would be no convenient place to put them, and they would easily be damaged (or personnel would learn to bypass them).
In most applications, rechargeable batteries are not suitable for a power supply. Rechargeable batteries not only impose a user burden (to perform recharging), but also tend to have electrical characteristics which may be dependent on the discharge/recharge history of the particular battery. Many possible applications cannot tolerate such uncertainty, and require a degree of reliability which demands a very conservative approach to power supply design and rating.
The need to conserve power actually implies several separate constraints: the consumption requirements of both the active and the standby mode must be separately minimized, and the issues to be considered are somewhat different.
The most difficult issues are presented by the standby mode. The portable module cannot afford the power to continually broadcast a beacon, but, even if the base station module broadcasts a beacon to ascertain the possible presence of receiver modules, the power requirements of listening for such a beacon are large.
Suppose, for example, that a portable data module, with non-rechargeable batteries, is desired to have a lifetime of at least 10 years, and to be able to perform at least 1,000,000 data transactions during its lifetime. (This is an extremely aggressive set of specifications, and is believed to be far beyond the capabilities of any system presently available.) Suppose further that the available battery energy is 2000 Joules (1 milliAmpere for 190 hours at 3 Volts). Then the power dissipation in the standby mode must be no more than several millionths of a Watt, or all of the battery energy will be dissipated merely in waiting for the active communication transactions to begin, before the design lifetime has expired. The present application discloses several novel teachings which are directed to this aspect of power conservation.
Some previously proposed methods for implementing such wireless-access data systems have used passive components for RF detection, connected so that the RF power received from the base station can actually provide the necessary power to operate the remote module. Such systems require that the RF power level at the receiver must be far higher than would be needed merely for communication.
The present application discloses a receiver architecture which is not only advantageous in a portable wireless data module, but can also be used in a wide variety of other communications receivers where low power consumption is needed (and particularly where low standby power consumption is needed).
A micropowered RF receiver is provided, which uses a comparator (or comparators) at its input terminals. Preferably no analog gain stages are used, either before or after the comparator, and the comparator's input terminals are directly connected to an all-passive antenna circuit. This provides reasonable sensitivity, but does not consume large amounts of power in the standby mode.
A further subclass of embodiments uses feedback from pulse-counting logic to adjust the gain of the comparator(s) which receive the input.
The length of incoming pulses is measured, in a digital stage of the portable module, by a counter. When this counter reaches a count which is much longer than would be expected from any of the allowed set of symbols, it provides an overflow-indicating pulse to control logic which reduces the bias current supplied to the comparators 420 at the input to the receiver circuits, and thereby reduces the sensitivity of these comparators.
The current source in the input comparators in the receiver circuits is the primary location of standby power dissipation in the presently preferred embodiment. Therefore, the size of this current source is a significant parameter in designing the remote module: if the comparators are redesigned to draw more current, the sensitivity of the receiver circuits will be increased, but the battery lifetime will be decreased. The total charge available in the battery, and the desired design lifetime, set a limit to the current which can be drawn. The minimum current draw is defined by the desired sensitivity of the receiver: the desired minimum sensitivity will dictate a certain level of current to achieve it.
Thus, battery lifetime considerations indicate a maximum size for the current source, but do not indicate a minimum size. Therefore, in this class of embodiments, the channel width of the current source device is divided up between several devices in parallel. For example, where the maximum device width is W, this device width can be allocated into four parallel current-source devices having widths of W/15, 2W/15, 4W/15, and 8W/15. A switching transistor is placed in series with each of these current-source devices, and a four-bit down counter is used as a current-source-control counter. The four output bit lines of the current-source-control counter are each connected to one of these switching transistors, to control one of the current-source devices.
Thus, when the main counter saturates, the current-source-control counter is decremented, and the total channel width of the current sources in the input comparators of the receiver circuits is reduced. This reduces the sensitivity of the comparators. Pulse counting continues, and if saturation occurs again the receiver sensitivity is decremented again.
A further feature of this class of embodiments is the use of a slow timer circuit to recover from saturation. A simple RC timing circuit with a long time constant (e.g. 1 to 10 seconds) is used to periodically reset the current-source-control counter. This assures that the portable module will be able to rapidly recover from saturation.
All antennas tend to have higher gain in some directions than in others. The directions where gain is maximal are referred to as "lobes," and the directions where antenna gain is zero or minimal are referred to as "nulls." The presence of antenna nulls could cause the portable module to fail to receive signals from the base station, if the portable module's orientation happens to be wrong. To avoid antenna nulls, the wireless data modules may includes two micro-antennas oriented to avoid coincident nulls. However, the RF signals cannot be directly combined, or the two antennas will simply act as one combined antenna, with a new set of possible nulls.
The presently preferred embodiment uses two separate comparators, connected to separate antenna inputs, to detect the presence of an RF signal. One of the two comparators is given preference, but, if no pulses are being detected by the primary comparator, the output of the secondary comparator is monitored. (Thus the two comparator outputs are combined in what is almost an OR relation, except that, once an incoming pulse train has been detected by one comparator, interference by the other comparator is avoided.)
Note that U.S. Pat. No. 4,584,709 to Kneissel et al. discloses a portable radio which switches between multiple antennas until an antenna with adequate signal quality is found. The configuration disclosed in this patent uses separate tank circuits for the two antennas, with PIN diodes used to switch the tank circuits (and therefore the antennas) in and out. Timesharing is used to monitor the signals on the two antennas, to see which is better, and the signal from the better antenna is connected to the receiver. (By contrast, the presently preferred embodiment does use timesharing in this fashion.)